Attempts have been made to successfully bond a metal strip connector to an electrode incorporating a foam type substrate for an electrochemical cell, particularly applicable to electrochemical cells having an alkaline electrolyte with at least one of the electrodes taking the form of a porous metal substrate containing active material. Typically, such substrate functions as a porous support or lattice structure, and wherein similar to a sponge, voids are interconnected in a three-dimensional network classically referred to as "foam". Such lattice structure substrate prior to being filled with active material has a porosity in excess of 95%. After filling of the pores with active material, the electrode is compressed to a design thickness to facilitate assembly and proper cell operation after manufacture of the cell.
In investigations undertaken by the applicant and the corporate assignee of this application, it was determined that considerable problems arise when connecting nickel connector strips, conventionally called "Ni-tabs" to the foam type substrate electrodes, conventionally termed "Ni-foam electrodes". The investigations resulted in an appreciation that it is not possible to weld onto foam containing active material. Attempts made to ultrasonically weld the Ni-tab directly to the Ni(OH).sub.2 pasted foam failed due to the resistivity of the paste. In the investigations, effort was concentrated on the removal of a Ni(OH).sub.2 paste off a Ni-foam porous substrate and to perform the weld. Air jets applied locally on a limited area of the Ni-foam substrate at 100 psi compressed air pressure failed to remove the paste. Initial efforts at ultrasonic paste removal were also unsuccessful.
Subsequent to the making of the claimed invention herein, the applicant became aware of the existence of U.S. Pat. No. 5,086,969, issued Feb. 11, 1992 and entitled "METHOD OF BONDING A METAL CONNECTION TO AN ELECTRODE HAVING A FOAM TYPE SUBSTRATE FOR AN ELECTROCHEMICAL CELL, AND AN ELECTRODE OBTAINED BY THE METHOD" to Jean-Marc Guerinault and Joel Brunarie of France, and assigned to SAFT of Romainville, France, the parent of the corporate assignee of this application, SAFT AMERICA, INC.
U.S. Pat. No. 5,086,969 recognizes a common bonding problem in this art and solves that problem by a different bonding method for bonding a metal strip connector to an electrode having a compressed foam type substrate filled with active material. The bonding method comprises forming at least one hole through the compressed active material filled foam type substrate and a surrounding zone, causing a metal tongue to penetrate superficially into a first face of the electrode and welding a metal strip connector to the metal tongue from the opposite face of the electrode, with welding being effected through the hole in the electrode. Preferably, the metal strip connector includes a boss sized to and received in the hole to facilitate welding of the connector to the tongue from the end of the boss penetrating the hole.
While such bonding of the metal strip connector to the foam type substrate is successful, it requires the incorporation of a separate metal tongue or backing member, thereby increasing the thickness and bulk of the connection at the substrate, it reduces the active area of the active filled porous substrate available and tends to stiffen the cell structure in the vicinity of the active area of the substrate.
It is therefore a primary object of the present invention to provide an improved, low cost, high-speed method of removing the Ni(OH).sub.2 active material within a localized area, preferably throughout a foam type substrate from one face to the other, which method is applicable to fully automated process manufacturing of electrochemical cells, which readily permits subsequent direct welding of a Ni-tab to the Ni(OH).sub.2 paste-free foam at the localized area of paste removal, which eliminates the need for a tongue or backing member and which neither destroys nor deforms the Ni-foam substrate in the localized area effected by the cleaning process and the subsequent welding or areas outside thereof, and which effects a connection between the Ni-tab and the Ni-foam electrode of satisfactory strength and of low electrical resistance.